The invention relates to a sensor with at least one silicon-based micromechanical structure, and to a method for producing such a sensor.
Sensors that have silicon-based micromechanical structures are known. If the micromechanical structure is movable element (sensor element), then such sensors can be used as acceleration sensors, rotary acceleration sensors, inclination sensors, resonant magnetic field sensors, or rotation rate sensors. Typically, these sensors comprise a foundation wafer, which is usually likewise formed from material that contains silicon, in which the structure is integrated into a so-called sensor chamber of its surface. To protect the structures and the atmosphere prevailing in the sensor chamber, the foundation wafer is covered with a cap wafer, with a covering that covers at least the sensor chamber. This cap wafer, because of its micromechanical prestructuring, has many individual caps joined together, of which each individual cap comes to rest exactly above the sensor chambers and is soldered to the sensor chamber in hermetically sealed fashion, and thus hermetically seals off the underlying sensor structure from the environment.
From German Patent Disclosure DE 195 37 814 A1, the production of such sensors is known. Based on a silicon substrate, insulation layers and conductive layers (in the form of electrodes or electrical connections) are applied in alternation, using the conventional method steps known from semiconductor technology. By means of masking and machining methods, also known, structuring of such layers can be done, for instance by way of lithography or etching processes. In an ensuing process step, a polycrystalline silicon layer (epipolysilicon), with a layer thickness ranging from a few nanometers to several tens of micrometers, preferably from 10 to 20 μm, is created. From this silicon layer, in the final analysis the required structures are etched out and made freely movable by underetching. The previously applied, structured, buried conduction layer makes it possible to establish electrical connections between elements of the sensors and the “outside world”, in the form of so-called connection regions. These connection regions, which are connected via the conductive layer to sensor elements, carry a metallizing on their surface. The connection region with the metallizing applied over it serves to secure bonding wires, with which an electrical contact with the structures in the sensor chamber (sensor structure) is then to be established. The sensor structure described in DE 195 37 814 A1 is distinguished by the fact that it has a movable (free-standing) region with measurement capacitors, where changes in the measuring capacitance upon a deflection are used as a measurement variable.
The components of the sensor, all described as examples here, will simply be called the foundation wafer, for the sake of simplifying the further description. The foundation wafer must be hermetically tightly joined to the cap wafer in a final machining step. To that end, in the prior art it is provided that a cap be secured above each sensor chamber to the surface of the foundation wafer by means of a glass solder layer on the cap wafer (known as the seal-glass solder process). A disadvantage of this is that this technology is relatively expensive. The glass solder layer must be applied to the micromechanically structured cap wafer by means of screen printing. The cap wafer must already be structured on both sides to enable the ensuing covering and contacting of the sensor; that is, the cap wafer itself is already intrinsically expensive. Moreover, this capping technique requires a relatively large amount of space, in which up to about 75% of the individual element area is required for anchoring the cap to the sensor chip. The resultant structural height and limited structuring options preclude the use of certain especially economical housings for the sensor.
Often, the free-standing regions covered by the caps of the cap wafer are relatively large. Sensor structures often have edge lengths of several hundred micrometers. If such a sensor is subjected to a mechanical overload, then in an extreme case, sagging of the cover layer can lead not only to interference with the sensor properties but in the final analysis also to an excessive deflection of the sensor structure, to the point of irreversible damage.